This invention relates to a device and method for heating a cellular or honeycomb structure used for emission control in order to initiate faster light-off in applications such as automotive catalytic converters, diesel particulate filters, wood stove, industrial stacks and any application in which the exhaust gas stream temperature is too low to initiate fast light-off of the catalytic converter. The invention also relates to a device and method for reducing the catalytic light-off time of a cellular or honeycomb structure when such structure is used as a catalytic converter for internal combustion engines.
Pollutants from automotive exhaust systems continue to present a substantial environmental challenge. Catalytic converters have been successful in reducing automotive exhaust pollution. However, as pollution standards become more stringent, there is an increasing need for better and improved converters. When a catalytic substrate or structure is heated to a sufficiently high temperature, the pollutants are catalytically reacted to produce harmless gases.
The Federal Test Procedure (FTP) is an emission certification test used on light-duty vehicles. Cold-start under FTP standards is the starting of an engine after a period of 12 to 36 hours of non-operation in a temperature environment of 20.degree.-30.degree. C. For most cold-starts, there is a time span of one to two minutes between the time the emissions begin, that is, cold start, to the time the substrate heats up sufficiently for catalyst "light-off" to occur. It is estimated that about 50% of the pollutants escaping into the atmosphere from a vehicle equipped with a catalytic converter is generated in these first two minutes following cold start-up. Light-off time is the time it takes to achieve a 50 percent conversion efficiency. The temperature at which about 50% of the pollutants have been converted to harmless gases is the light-off temperature. For most emission control systems, the light-off temperature is generally in the range of about 250.degree. to 350.degree. C. By reducing the light-off time, the amount of pollutants escaping into the atmosphere can be reduced.
Modifications have been made to catalytic converters to increase the rate of heating in order to obtain shorter light-off times. Numerous schemes have been proposed to provide faster light-off of catalytic converters and thereby reduce the amount of pollutants. Mainly, it has been suggested to heat the entire catalytic converter or to heat the inlet gas stream in order to bring the substrate to its light-off temperature faster. Such heating methods are inefficient and require considerable amounts of energy. According to one of these modifications, a heater is disposed in the exhaust gas passage ahead of the catalytic converter. The suggested system consists of two separate sections where one section is a metal substrate catalyst having no heating ability, and the second section is a catalyzed metal substrate with the ability to be heated electrically. The heater first heats up the exhaust gas which then heats up the catalytic substrate. The problem with this method is that a substantial amount of electric power is required to raise the metal substrate to a temperature high enough to heat all of the incoming exhaust gases. Also, the efficiency of this method is low because a significant amount of heat is lost between the heater and the converter. It is not known to preheat a cellular structure or honeycomb to provide an early light-off catalytic converter by forming a resistance heater directly onto the surface of the structure and providing means for activating the surface by passage of electrical current.
Accordingly, it is the object of the present invention to provide a cellular structure for quick light-off by forming a resistance heater on the surface of the structure. The present invention discloses a self-heated catalytic converter suitable for various applications including automotive emissions control, diesel fumes, wood stove, industrial stacks emission and any application in which the exhaust gas stream temperature at start-up is too low to initiate fast light-off.